1. Field of the Invention
The present invention relates to a method for producing 2-hydroxy-4-(methylthio) butyric acid (HMBA) an analog of the essential amino acid methionine, by fermentation. Fermentation is a biological process wherein a microorganism, using the carbon, sulphur and nitrogen provided in the culture medium, bio-synthesizes a product of interest that is usually chemically synthesized.
2. Description of Related Art
2-Hydroxy-4-(methylthio) butyric acid (HMBA), commonly referred to as “hydroxymethionine” is an analog of the essential amino acid methionine, and an important feed additive. It is commonly used in poultry diets because methionine in commercial corn-soybean-based feedstuffs is considered to be the first limiting amino acid.
The methionine hydroxy analog contains a hydroxyl radical on the α-carbon of the methionine molecule rather than an amino group. HMBA has the formula:CH3SCH2CH2CH(OH)COOHIn contrast with the amino acid, it is not used directly by the organism in protein synthesis, because it must be anabolically converted into the amino acid to be used as such. HMBA is not used in the pure form, but in various forms, namely:                a mixture of calcium and ammonium salts of HMBA (U.S. Pat. No. 2,745,745 and U.S. Pat. No. 2,938,053),        acidic aqueous solutions (U.S. Pat. No. 4,353,924),        calcium salts of HMBA, obtained by the process described in U.S. Pat. No. 3,175,000.The preparation of HMBA by a chemical route has been known for a long time. Several patents from NOVUS International (PCT/US98/01595), MONSANTO Company (EP0142488), BRITISH Telecomm (EP0143000) or Rhone Poulenc Animal Nutrition S.A. (U.S. Pat. No. 6,180,359) describe hydrolysis of 2-hydroxy-4-methylthio-hydroxybutyronitrile (HMBN) into HBMA by a two-stage process. All these technologies rely approximately on the same raw material and key intermediates.        
The first stage consists in bringing the 2-hydroxy-4-methylthiobutyronitrile (HMBN) into contact with strong inorganic acid such as hydrochloric or sulphuric acid. In a subsequent stage, after dilution with water, the hydrolysis is completed at a higher temperature. The HMBA is then extracted with organic solvent which is not very miscible with water, such as ketone, and then the solvent is removed by evaporation.
The amide 2-hydroxy-4-methylthio-butyronitrile (HMBN) is synthetized by reaction between methyl-mercapto-propionaldehyde (MMP) and hydrocyanic acid (HCN) or sodium cyanide (NaCN).
During the past few years, new methods have emerged involving enzymes or biological material. Aventis Animal Nutrition S. A. has for instance described and patented a method for the preparation of HMBA by enzymatic hydrolysis of the 2-hydroxy-4-methylthiobutyronitrile intermediate. The invention is based on bioconversion of HMBN after contacting the molecule with immobilized biological material having nitrilase activity (U.S. Pat. No. 6,180,359). A similar process was described by Novus with the enzymatic conversion of 2-hydroxy-4-(methylthio)-butanenitrile to 2-hydroxy-4-(methylthio)-butaneamide or 2-hydroxy-4-(methylthio)-butanoic acid or salts (WO 1998032872).
In their efforts to improve the production of methionine by microorganisms, inventors have surprisingly found that hydroxymethionine can be also produced in microorganism from a simple carbon source in a fermentative process. This is the first report of a fully biological production of methionine hydroxy analog.